Should I invest a $15,000 windfall at once or spread it out?

Comparing investing a $15,000 windfall immediately versus spreading it evenly over the next 12 months, both at the same average return.

This compares two mechanical strategies at one assumed average return — it says nothing about what the market will actually do next. Spreading it out is often chosen for the psychological comfort of not investing at a single, possibly bad, moment.